This invention relates generally to insertable saw teeth for circular saw blades as utilized in commercial sawmills, and more particularly to a new and improved, one piece, insertable saw tooth which provides numerous advantages over the prior art two-piece inserts, such as, it is self-centering when inserted onto a saw blade, eliminates of any movement and working between the bit and the shank, provides greater strength and tension to the rim section of the saw blade, is less prone to causing damage to the saw blade holding the insert, is safer than the prior art two-piece saw tooth, can be inexpensively tipped and retipped to provide an exceptionally long life span as well as other advantages discussed below.
Practically all circular saws utilized in commercial sawmills utilize saw blades which have replaceable saw teeth. Because such commercial circular saw blades are relatively large, having diameters up to 60 inches, they are relatively expensive. Therefore, when the saw teeth become damaged to the point where they cannot be reshapened, it is far less costly to merely replace the damaged teeth rather than replacing the entire saw blade.
Cut-off saw blades, i.e. those used for cutting logs to length and therefore cut across the grains of the lumber, usually utilize insertable saw teeth which comprise elongated one-piece inserts which merely slide into a mating elongated socket on the saw blade. While the inserts are dimensionally proportioned to fit rather snugly within the sockets, it is further essential that the inserts be riveted to thereby lock the inserts into the saw blade sockets.
Headrig saws, i.e. those used for the breakdown of logs which normally cut the log parallel to the grains, as this invention pertains to, all utilize a two piece saw tooth insert, which comprise a bit, which in part forms the cutting edge of the saw tooth, and a shank, which serves to lock the bit in place within a circular socket on the periphery of the saw blade, thereby eliminate the need of a rivet. There are primarily only two basic types of headrig saws and inserted teeth therefor in commercial use, namely the single circle and double circle types, with differing sizes of the two basic types. The circular nature of the socket permit the bit and shank to be rotated thereinto with an end portion of the shank biased against the bit to hold it in place.
The above noted two piece saw tooth inserts have been successfully used for well over half a century, providing the benefit for which they were designed. While there have been a number of minor design changes, normally for use in unusual applications, such as the cutting of frozen lumber and the like, the basic design has remained virtually unchanged since the 1920's. Despite the fact that the design has proven itself to be rather successful, there are problems that are encountered when using such two piece inserts. Most notable of these problems result primarily from the two piece nature of the insert, in that use of the saw blade will naturally cause some relative movement between the bit and shank. This movement may cause the bit to creep radially outward to adversely effect the cutting force of the saw. On the other hand, such movement may cause the edge of the saw blade to bend at the shoulder, with the result that the saw blade is pulled one way or the other, seriously effecting the straightness of the cut. It is also know that the weakest line in the saw blade shoulder is at the joint between the bit and the shank, so that relative movement between the bit and shank, which causes bending of the shoulder, may in fact even cause the shoulder to fracture, which not only causes serious damage to the saw blade, but can be a very hazardous situation as the high speed motion of the fractured shoulder and both saw tooth inserts will cause them to be propelled in practically any direction. In view of this movement between the bit and shank, which naturally results, it is necessary that the bit and shank inserts be periodically reset, which, of course, means additional downtime.
Another difficulty caused by the two piece inserts, is that they do not always align themselves properly when inserted into the saw blade. Despite the fact the edges of the two pieces and the edges of the saw blade sockets are provided with mating "V"s and "V" grooves, the two piece inserts can and do "hinge" to some degree within the saw blade socket, or may otherwise be improperly aligned when inserted. Indeed, when such inserts are first inserted into a saw blade, it is required practice that the broad-side surfaces of the inserts be tapped with a hammer to properly seat the "V"s into the "V" groves and otherwise assure that the two pieces are properly centered with reference to the saw blade and to each other. Even when such inserts are properly aligned, they can become misaligned during service, as noted above, due to movement between the two pieces.
Still another disadvantage of the two piece saw tooth inserts, is the fact that they do become dulled rather rapidly, and as a result have to be resharpened quite frequently. While some bits have been provided with hard cutting inserts, such as carbide tips soldered thereto to form the cutting edge, the design of the bits are such that they are not readily amenable to carbide tipping as there is little steel surface to back up and support such a tip, and the bits are such small pieces to work with that the cost of tipping or retipping is rather high. In fact the cost of retipping carbide tipped bit is so high, that most commercial mills do not find them to be cost justified. Accordingly, most mills do not utilize hard tipped bits.
A further disadvantage of the prior art saw tooth inserts is that because insertion and removal of the insert assembly causes wear on the saw blade sockets, that the saw blade sockets do wear down, somewhat enlarging the radius of the circular socket. This then necessitates that the saw tooth supplier provide shank portions in a variety of slight oversizes for use with those saws in which the sockets have worn larger as a result of repeated insertion and removal of the inserts over a period of years.
An ever further disadvantage of the prior art saw tooth insert assemblies is that they do not provide uniform and consistent tensile forces on the rim of the saw blade to the extent desired. That is to say, rotational forces and frictional heat, causes the peripheral portion of the saw blade to expand to a greater extent that the portions radially inward therefrom. In the absence of any corrective effort, this action would tend to put the rim of the saw blade in compression, with the result that the two areas of the saw blade are not in a state of stress equilibrium during operation. When such a deviation from equilibrium does happen and becomes excessive, the saw blade will tend to warp or buckle, so that it will not cut in a straight line. Because of this phenomenon, it is common practice to physically work, i.e. compress, the inner portions of the saw blade so that it is in a static state of compression, and therefore, the rim portion is put in a static state of tension, when the saw blade is not in use. Subsequent use of the saw blade in service, however, will heat the rim portion, as noted above, so that the static forces within the saw blade will approach equilibrium. When the two piece saw tooth inserts are utilized, the expansive action of the inserts compressed into the sockets often causes a variable compressive force in the rim of the saw blade so that the desired tensile forces in the rim are not usually as uniform as desired. Moreover, because of the nature of the inserts, and the relative movement that occurs therebetween, as discussed above, the tensile forces in the rim of the saw blade will vary during use from one insert to the next, over even a short period of time.